Human and animal studies indicate that bilirubin monoglucuronide (BMG) may play a central role in the formation of gallstones (GS). Significantly higher gallbladder concentrations of BMG have been found in patients with GS than in patients without GS. Disorders associated with an increased BMG production (e.g. hemolytic anemias or glucuronyl transferase deficiency) are associated with an increased excretion of BMG and an increased incidence of GS. The guinea pig, hamster, deer mouse and prairie dog, all models for GS formation, excrete BMG as the major pigment in bile. Unconjugated bilirubin and BMG have been associated with biliary sludge, perhaps the first step in GS formation. Three mechanisms may account for these relationships. Firstly, because of the presence of only one polar glucuronic acid, BMG may precipitate in bile because it is less soluble than the major pigment of human bile, bilirubin diglucuronide (BDG). Secondly, BMG may undergo spontaneous hydrolysis in alkaline bile to form UCB and glucuronic acid. Thirdly, BMG easily undergoes molecular rearrangement to BDG and very insoluble unconjugated bilirubin (UCB) -- the nidus for most cholesterol GS and the major tetrapyrrolic component in many pigment GS. To test these hypotheses, I will systematically study in inert polyethylene eppendorf test tubes the stability and solubility of pure BMG (purified from rat bile by the HPLC method of Spivak and Carey) over a range of physiologic pH in various physiologic concentrations of bile salts, cholesterol, phospholipids and proteins at 37 C. A novel 10 minute HPLC method will allow us to serially measure concentrations, of BMG in the supernate and precipitate and to detect the formation of BDG and III-Alpha, XIII-Alpha and IX-Alpha isomers of UCB from BMG. Since the IX-Alpha isomer of BMG is the only pigment present initially, formation of predominantly IX-Alpha UCB will indicate BMG hydrolysis while formation of the III-Alpha and XIII-Alpha isomers of UCB and BDG will indicate molecular rearrangement of BMG. These solubility, hydrolysis and rearrangement studies of BMG should yield crucial information about the physical-chemical phenomena in gallbladder pigment precipitation.